1. Field of the Invention
The invention relates to control of power from an engine in a hybrid electric vehicle powertrain wherein engine power demand is determined by road-load.
2. Background Art
In a hybrid electric vehicle powertrain (HEV), particularly a power-split hybrid electric vehicle powertrain with a divided power flow path from an engine and an electric motor, the path has two power input gear elements and one power output element. Two of the gear elements have definite rotation directions and magnitudes, which determine the rotation direction and magnitude of the third element. One of the elements can be kept at a constant speed, while the other elements can change. This is a characteristic that is similar to a characteristic of a conventional continuously variable transmission. Unlike a conventional continuously variable transmission, however, the torque relationship of the gear elements is fixed even though the speed relationship can be changed.
In a so-called power-split hybrid electric vehicle powertrain of the kind shown, for example, in U.S. Pat. No. 6,994,360, selection of engine speed and operation of the engine throttle is based upon an engine power demand. This engine power demand is calculated by a vehicle system controller and is based primarily upon a driver demanded wheel power. Typically, driver demanded wheel power is measured by accelerator pedal input, or an input from a speed control system. It is based also on a high voltage battery state-of-charge. Given an engine power demand, the vehicle system controller selects an engine speed and an engine torque to achieve engine power demand while optimizing fuel economy and minimizing vehicle emissions.
The CVT-like nature of a power-split HEV allows the vehicle to operate in a significant range of engine speeds at any given vehicle speed. As a result of the optimization for fuel economy, the targeted engine speed is quite sensitive to small changes in driver demanded wheel power. However, transient engine power demand conditions caused by frequent, small changes in engine operating point may reduce fuel economy, increase vehicle emissions, and result in a negative driver perception of engine speed hunting or instability.
Transient conditions may be reduced by filtering the engine power demand with a simple filter. However, such a filter will also negatively affect vehicle performance by delaying the engine's response to an acceleration or deceleration request.